1. Field of the Invention
The subject invention relates to a method and apparatus for processing a pseudoperiodic analog signal. Such processing is particularly advisable when an analog signal is converted into a succession of steep-sided pulses by a threshold switching circuit, with a view to use in a digital logic processing system.
2. Description of the Prior Art
Problems are frequently encountered when converting a periodic or pseudoperiodic analog signal into steep-sided pulses of constant amplitude. As an example, the situation may be encountered where it is necessary to generate a train of pulses from an analog signal, each pulse being generated in response to and in synchronization with a positive-going (or negative-going) part cycle of the analog signal which is considered to be "significant". It is known however that when employing analog to digital conversion techniques which employ a threshold switching circuit, the length of a pulse generated from what are termed "significant" part cycles become larger as the amplitude of the corresponding significant part cycle becomes higher. If the amplitude is too small, the corresponding pulse may be too short to be "recognized" and taken into account by a digital logic system or it may not even be generated if the peak value of the part cycle is below the threshold level of the threshold switching circuit.
To give a more specific example, there are certain known devices for reading information (i.e. check readers used by banks) which operate by detecting the passage of bars of magnetized ink in front of a magnetoresistive reading head. A reading device of this kind is described in particular in a French patent application No. 78.29848. The individual analog signals emitted by magnetoresistors in response to the passage of a signal bar is formed successively by a negative-going part cycle of low amplitude, a positive-going part cycle of high amplitude and a negative-going part cycle of low amplitude. The electrical energy corresponding to the positive part cycle is equal to the sum of the electrical energies corresponding to the two negative part cycles. The positive part cycle is thus considered to be the "significant" part of the cycle, since there is only a signal positive part cycle for each bar. At the reading magnetoresistor, a closely spaced series of bars produces a pseudoperiodic analog signal resulting from the sum of the individual responses of the kind described above. In the pseudoperiodic signal, each positive part cycle is "significant". Therefore, in principle, it appears that all that would be necessary would be to apply this analog signal to a threshold switching circuit through a simple diode in order to obtain a pulse train to be used by the digital logic system. However, the amplitude of each significant part cycle corresponding to a bar may vary within fairly wide limits in the pseudoperiodic signal since the bars (in particular as they appear on a check) are not necessarily of the same height or the same residual magnetism. The worst case is when an individual signal of high amplitude is generated immediately after an individual signal of low amplitude. In this case, the first negative part cycle of the individual signal of high amplitude is "substracted" from the significant positive part cycle of the individual signal of low amplitude and the steep-sided pulse which is generated from the positive half part cycle of reduced size is then of very brief duration or even, in some cases, completely non-existent.